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Microbial Communities in Serpentinites Make the Best of a Difficult Situation

29 March 2017

Serpentinite environments, like the Lost City hydrothermal field in the mid-Atlantic, have received much attention in recent years as potential cradles of life on Earth, and as analogs for sites that could support life on Mars. Microbes thrive on compounds released by reactions between serpentinizing minerals and seawater in hydrothermal vents. The submarine location of these sites, however, presents many challenges for scientists trying to study them.

The Coast Range Ophiolite Microbial Observatory (CROMO), located in northern California, is a serpentinization site that once lay at the bottom of the seafloor. Its current landlocked location allows researchers to study the geochemistry and microbiology of a serpentinization site in great detail. In a new paper, DCO members Katrina Twing and William Brazelton, (both at University of Utah Salt Lake City, USA), Matthew Schrenk (Michigan State University, USA), Michael Kubo (SETI Institute, USA), Dawn Cardace (University of Rhode Island, USA), Tori Hoehler (NASA Ames Research Center, USA) and Tom McCollom (University of Colorado Boulder, USA) describe the geochemical factors affecting microbial communities at seven wells drilled into the freshwater aquifer within serpentinite rocks at CROMO. While the site provides easy access for scientists, it is a difficult environment for microbes. The study appears in the DCO Early Career Scientist research topic of Frontiers in Microbiology.

“CROMO acts as a window into the ocean crust, but we can access it much more easily and less expensively than going out to the middle of the ocean,” said Twing. “For the first time, we were able to constrain the geochemistry and biology of this system by directly comparing a breadth of different conditions.”

Serpentinization in the freshwater aquifer creates highly alkaline fluids that are low in inorganic carbon but contain dissolved methane and small organic molecules such as acetate. These sites are rich in hydrogen but lack oxygen and other electron acceptors that microbes use for metabolism. These electron accepters are the equivalent of the oxygen that humans need to breathe. Due to these limitations, CROMO supports only a small number of bacterial species.

“We found incredibly low diversity there. The highest pH site had two bacterial species that made up about 75 percent of the sample. To have a system with such low diversity is virtually unheard of,” said Twing.

One dominant species is closely related to the Betaproteobacteria Hydrogenophaga pseudoflava, which occurs at other serpentinite sites worldwide. A handful of Clostridia species, which are especially hardy, spore-forming bacteria, also predominate. While the researchers aren’t entirely sure what role the Clostridia play in the system, they suspect they can perform multiple functions. The metagenome sequenced from fluids revealed bacterial genes required for hydrogen, methane, acetate, and carbon monoxide metabolism.

Surprisingly, the researchers detected no Archaea in the fluids at CROMO. In marine hydrothermal vent systems, Archaeal species that generate or consume methane dominate vent chimney ecosystems.

By comparing the bacterial communities with the geochemistry at each well within CROMO, the researchers used statistical tests to pinpoint methane, carbon monoxide and pH as important factors driving bacterial diversity at the site.

In future work, Twing plans to examine the drill cores collected at CROMO to look for further bacterial diversity within the rocks.

As CROMO was one of the earliest Deep Life pilot projects, with drilling beginning in 2011, Twing notes that her experience sampling at the site was valuable as a dry run for a later trip. In 2015 she joined a research cruise with Schrenk and other colleagues from the International Ocean Discovery Program, to collect serpentinite cores from the Atlantis Massif, located next to the Lost City hydrothermal field.

Images:

Top: Drilling operations to establish the CROMO wells began in August 2011. Credit: Matthew Schrenk Center: The water sampling set up at the CROMO wells. Credit: Katrina Twing Bottom: Katrina Twing and collaborators deploy down-well experiments into a CROMO well. Crystal George, formerly of East Carolina University, and Mike Kubo of the SETI Institute, stand in the background. Credit: Melitza Crespo-Medina

The Deep Carbon Observatory (DCO) is a global community of multi-disciplinary scientists unlocking the inner secrets of Earth through investigations into life, energy, and the fundamentally unique chemistry of carbon.